The present invention relates to an image-forming apparatus such as a copying machine or a facsimile and, more particularly, a control apparatus for controlling the document-scanning velocity of such an image-forming apparatus.
In a copying machine of the document fixed type, a document set on a document table is exposed and scanned with an exposure lamp. An electrostatic latent image of the document is formed on a photosensitive drum as a photosensitive body with light reflected from the document. The photosensitive drum is rotated at a predetermined velocity. The scanning velocity of the exposure lamp is proportional to the rotational velocity of the photosensitive drum in accordance with a proportionality constant corresponding to copying magnification. A scanning velocity control circuit has been developed which detects the rotational velocity of the photosensitive drum and controls with reference to the detected velocity. A conventional circuit of this type is illustrated in FIG. 1.
A rotational velocity of a main motor 81 for driving a photosensitive drum 80 is detected by a velocity detector 82. An output signal from the velocity detector 82 is supplied to a frequency multiplier 83 and is multiplied with a coefficient to increase frequency. An output signal from the frequency multiplier 83 is supplied to a programmable frequency divider 84 to be frequency-divided thereby. The frequency divider 84 then produces a reference signal having a frequency corresponding to copying magnification. The reference signal is supplied to a phase comparator 85. A rotational velocity of a scanning motor 86 for driving a scanning means for optically scanning the document is detected by a velocity detector 87. An output signal from the velocity detector 87 is supplied to the phase comparator 85. The phase comparator 85 compares the phases of the reference signal from the frequency divider 84 and of the signal from the velocity detector 87, and supplies a comparison result signal to a driver 88. The driver 88 controls a drive signal to be supplied to the motor 86, in accordance with the output signal from the phase comparator 85. In order to achieve stable control, the frequency of the signal from the velocity detector 82 is increased by the frequency multiplier 83 and the frequency of the output signal from the frequency multiplier 83 is divided to provide a reference signal having a frequency corresponding to the copying magnification. The phase comparator 85 compares the reference signal with the signal from the velocity detector 87 so that the two signals have frequencies which are proportional to each other.
However, in the apparatus described above, a separate frequency multiplier and programmable frequency divider are used. Therefore, the number of circuit elements to be used is increased, and the resultant apparatus becomes expensive. Furthermore, since control is performed by changing the frequency of the reference signal in accordance with the copying magnification, the range of frequency change of the reference signal in accordance with the copying magnification is widened. The highest frequency at which the circuit can be operated is fixed. Then, a signal of low frequency may be used as a reference signal, so that stable control may not be performed.